Gas turbine engines typically include several stages including a fan, a compressor, a combustor, and a turbine. Some of these stages utilize rotating airfoils with shaped blades arranged in series. The blades convert thermal energy from the combusted gas into mechanical work used to turn a rotor. The blades positioned forward of the combustor are turned by the rotor to compress air entering the combustor.
Blades, including turbine blades in particular, can utilize a pocket recess which comprises a recess cavity that extends radially through the length of the blade. The pocket recess creates an opening at the tip of the blade. The pocket recess is used for efficiency purposes to reduce the weight of the blade and to reduce blade creep. During operation of the gas turbine engine, air flow enters and exits the pocket recess with rotation of the blade due to the law of conservation of mass.
During operation of the gas turbine engine, the blades have one or more harmonic frequencies that coincide with integer multiples of the blades rotational frequency (also called the blade pass frequency). If the blade reaches one of these harmonic frequencies, the blade will become excited and vibrate. Additionally, during engine operation various aero-excitation source frequencies can be created as air passes over components of the gas turbine engine including the blade. These source frequencies can be transmitted to the air, causing unsteady fluid pressure oscillations, which can be transmitted to the blade. If a blade resonance frequency coincides with an aero-excitation source frequency, an excitation occurs causing undesired vibrations in the blade.
The tip leakage flow is induced by a pressure difference between the pressure at the pressure surface of the blade and the pressure at the suction surface of the blade. This phenomenon is also true for blades that employ the pocket recess. The leakage flow over the blade pocket recess can excite and sustain a longitudinal aero-acoustic mode resulting in pressure fluctuations within the pocket recess and result in the generation of a loud tone noise of high sound pressure levels.
In addition to the generation of noise, a blade employing the pocket recess will experience aero-acoustic-mechanical coupling phenomenon if one of the natural frequencies of the blade coincides with the aero-acoustic pressure oscillation frequencies as a result from air entering and leaving the pocket recess. If such a coincidence occurs, force on the walls of the pocket recess (caused by acoustic pressure in the cavity along the wall interface) supplies energy that sustains blade vibrations. At the same time that blade vibrations are sustained, the acoustic pressure field in the cavity is strengthened by blade vibrations along the pocket wall interface. As a result of these phenomenon, blades can be become excited, damaged, or fail (in extreme instances) due to the force of resonance.